1. Field of the Invention
The present invention relates to stacked-layered thin film solar cells and, more particularly, to a stacked-layered thin film solar cell with a light-absorbing layer having a band gradient and comprising indium and aluminum.
2. Description of the Prior Art
Solar cells are p-n junction semiconductor devices that directly generate electricity from sunlight through a process known as the photovoltaic effect. To meet the demand for thin solar cells, stacked-layered thin film solar cells have heretofore been a trend for the solar industry. The most important advantage of stacked-layered thin film solar cells is that they are cost-competitive, though problems with their efficiency and stability remain unsolved.
At present, the light-absorbing layer of a stacked-layered thin film solar cell is mostly made of amorphous silicon, copper indium diselenide (CuInSe2, often abbreviated as “CIS”), or copper indium gallium diselenide (CuInGaSe2, often abbreviated as “CIGS”). It is because the aforesaid materials are semiconductor materials with a direct band gap and, more particularly, have the following advantageous features: a band gap value that matches most of the solar spectrum, a high absorption coefficient, and being capable of creating a p-n junction by modification of their compositions.
Referring to FIG. 1, a CIS stacked-layered thin film solar cell 1 includes a plurality of thin-film layers, namely a substrate 10, a back electrode layer 11, a light-absorbing layer 12, a buffer layer 13, a window layer 14, and a top electrode layer 15, arranged from bottom to top. The CIS stacked-layered thin film solar cell 1 is made essentially of chalcopyrite, which is a group I-III-VI compound and exhibits high resistance to interference and radiation, and therefore leads a long service life. CIGS stacked-layered thin film solar cells, on the other hand, evolved from CIS stacked-layered thin film solar cells by replacing, in part, the element indium thereof with the element gallium. CIGS stacked-layered thin film solar cells absorb light whose energy ranges from 1.02 ev to 1.68 ev, depending on the indium and gallium content of the solar cells, and are more efficient than CIS stacked-layered thin film solar cells in light emission.
Nowadays, the light-absorbing layers of CIS stacked-layered thin film solar cells and of CIGS stacked-layered thin film solar cells are usually manufactured by sputtering deposition, vapor deposition, or electroplating. In this regard, a conventional manufacturing process involves sputtering copper, indium, and gallium so as to make a film, and then selenizing the film. Alternatively, another conventional manufacturing process involves co-vapor deposition of copper, indium, gallium, and selenium, which is more efficient but also more complicated.
The element indium and the element gallium used in manufacturing CIS stacked-layered thin film solar cells and CIGS stacked-layered thin film solar cells, respectively, are precious metals which incur high costs to the detriment of mass production. Also, band gap discontinuity between the light-absorbing layer 12 and the buffer layer 13 remains an unsolved problem. Hence, there is still room for improvement in the prior art.